RE- the battery – I would pop it in the house and charge ad-hoc – keep in small plastic crate to avoid acid spills.
Permanent connection to charger – not really – not for months – more of a safety thing really especially if there is any condensation risk.
Regarding the fuel – the stabilisers are definitely worth using.
For bikes that never get used in the rain the ethanol issue is less serious – apart from the melting rubber fuel pipes – the real issue is with bikes that get used in the wet – fuel fillers and vents can allow damp/water into tank – combines with ethanol in dead areas of the tank and rusts out the bottom/seams. possibly not an issue with the GoldWing as the tanks is under the seat.
Sounds like you are on top of the beast with changing belts etc.
Possibly add fork oil to the list if its not done.